Balanced Mach-Zehnder modulator

ABSTRACT

An apparatus for modulating a beam of light with balanced push-pull mechanism. The apparatus includes a first waveguide comprising a first PN junction on a silicon-on-insulator substrate and a second waveguide comprising a second PN junction on the silicon-on-insulator substrate. The second PN junction is a replica of the first PN junction shifted with a distance. The apparatus further includes a first source electrode and a first ground electrode coupled respectively with the first PN junction and a second source electrode and a second ground electrode coupled respectively with the second PN junction. The apparatus additionally includes a third ground electrode disposed near the second PN junction at the distance away from the second ground electrode, wherein the first ground electrode, the second ground electrode, and the third ground electrode are commonly grounded to have both PN junctions subjected to a substantially same electric field varied in ground-source-ground pattern.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to U.S.patent application Ser. No. 14/640,903, filed Mar. 6, 2015, all commonlyassigned and hereby incorporated by references for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to electro-optic modulation devices andmethods. More particularly, the present invention provides an improvedsilicon-based Mach-Zehnder modulator and methods for balancing twoMach-Zehnder arms based on silicon photonic design platform.

Over the last few decades, the use of communication networks exploded.In the early days Internet, popular applications were limited to emails,bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Over the past, high data rate communication has been widely implementedvia optical network, in which data signals are carried by laser lightthat is specifically modulated using various kinds of electro-opticmodulators. Mach-Zhedner modulator is a widely used electro-opticmodulator often driven by a push-pull driver (or differential driver)for relieving swing voltage requirements and power consumption. Forpush-pull (or differential) drive, the balance between two Mach-Zehnderphase modulation arms is very important. But the performance ofconventional modulator is sensitive to unbalance of two modulation armscaused by implant mask misalignment due to the use of mirror symmetricimplant mask and doping profile (i.e. ‘p-n n-p’ or ‘n-p p-n’).Therefore, an improved Mach-Zehnder modulator and methods for balancingtwo Mach-Zehnder modulation arms based on silicon photonics platformregardless of implant mask misalignment are desired.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to electro-optic modulation devices andmethods. Various embodiments of the present invention provide asilicon-based Mach-Zhedner modulator with balanced two arms. Morespecifically, the invention provides a Mach-Zhedner modulator includingrepeated p-n or n-p doping pattern to maintain balance for twomodulation arms regardless of implant mask misalignment. In certainembodiments, the invention is applied for high bandwidth opticalcommunication, though other applications are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

Serial link performance is limited by the channel electrical bandwidthand the electronic components. In order to resolve the inter-symbolinterference (ISI) problems caused by bandwidth limitations, we need tobring all electrical components as close as possible to reduce thedistance or channel length among them. Stacking chips into so-called 3-DICs promises a one-time boost in their capabilities, but it's veryexpensive. Another way to achieve this goal in this disclosure is to usemultiple chip module technology.

In an example, an alternative method to increase the bandwidth is tomove the optical devices close to electrical device. Silicon photonicsis an important technology for moving optics closer to silicon. Forexample, electric signal can be converted to optical signal by one ormore silicon photonic-based electro-optic modulation devices in which asignal-controlled element exhibiting the electro-optic effect is used tomodulate a beam of light. In this patent application, we will disclosean improved electro-optic modulator utilized for high speedtelecommunication.

In a specific embodiment, the present invention provides an apparatusfor modulating a beam of light. The apparatus includes a first waveguidecomprising a first PN junction of a first p-type region interfaced witha first n-type region on a silicon-on-insulator substrate. Additionally,the apparatus includes a second waveguide comprising a second PNjunction of a second p-type region interfaced with a second n-typeregion on the silicon-on-insulator substrate. The second PN junction isa replica of the first PN junction shifted with a distance. Theapparatus further includes a first source electrode and a first groundelectrode coupled respectively with either the first p-type region orthe first n-type region of the first PN junction. Furthermore, theapparatus includes a second source electrode and a second groundelectrode coupled respectively with the second p-type region and thesecond n-type region of the second PN junction. Moreover, the apparatusincludes a third ground electrode disposed near the second PN junctionat the distance away from the second ground electrode. The first groundelectrode, the second ground electrode, and the third ground electrodeare commonly grounded to have both PN junctions subjected to asubstantially same electric field varied in ground-source-groundpattern.

In another specific embodiment, the present invention provides a methodfor manufacturing a linear Mach-Zhedner modulator with balanced arms.The method includes providing a silicon-on-insulator substrate andforming a first linear waveguide and a second linear waveguide in thesilicon-on-insulator substrate. The second linear waveguide is inparallel to the first linear waveguide with a laterally shifteddistance. Additionally, the method includes forming a first PN junctionin the first linear waveguide and a second PN junction in the secondlinear waveguide. The first PN junction and the second PN junction havea substantially identical repeated p-n p-n doing profile along onedirection of the laterally shifted distance. The method further includesforming a first ground electrode and a first source electrode to couplewith the first PN junction and forming a second ground electrode and asecond source electrode at the laterally shifted distance away to couplewith the second PN junction. Furthermore, the method includes forming athird ground electrode at another laterally shifted distance away fromthe second ground electrode beyond the second PN junction. The thirdground electrode is commonly grounded with both the first groundelectrode and the second ground electrode for imposing an electric fieldacross both the first PN junction and the second PN junction by asubstantially same ground-source-ground pattern.

In an alternative embodiment, the present invention provides a methodfor balancing two arms of a Mach-Zhedner modulator. The method includesproviding a silicon-on-insulator substrate. Additionally, the methodincludes forming a first linear waveguide and a second linear waveguidein the silicon-on-insulator substrate. The second linear waveguide is asubstantially same structure as the first linear waveguide shifted by adistance. The method further includes masking a first region of thefirst linear waveguide and a second region of the second linearwaveguide with repeated symmetry. Furthermore, the method includesimplanting p-type impurity to the first region and the second region.The method then includes masking a third region of the first linearwaveguide and a fourth region of the second linear waveguide withrepeated symmetry. Moreover, the method includes implanting n-typeimpurity to the third region and the fourth region such that the thirdregion interfaces with the first region to form a first PN junction inthe first linear waveguide and the fourth region interfaces with thesecond region to form a second PN junction in the second linearwaveguide. The second PN junction is substantially identical to thefirst PN junction.

In a specific embodiment, the method for balancing two arms of aMach-Zhedner modulator includes forming a first ground electrode and afirst source electrode respectively coupled to first region and thethird region associated with the first PN junction. The method furtherincludes forming a second ground electrode and a second source electroderespectively coupled to the second region and the fourth regionassociated with the second PN junction shifted with the distance awayfrom the first PN junction. Furthermore, the method includes forming athird ground electrode shifted with the distance away from the secondground electrode beyond the second PN junction. The third groundelectrode is commonly grounded with the first ground electrode and thesecond ground electrode such that each PN junction is subjected to anelectric field imposed with a substantially same ground-source-groundpattern.

The present invention achieves these benefits and others in the contextof known memory technology. However, a further understanding of thenature and advantages of the present invention may be realized byreference to the latter portions of the specification and attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIGS. 1A and 1B are simplified diagrams of typical Mach-Zhednermodulators with two arms formed by mirrored p-n implant masks.

FIGS. 2A and 2B are simplified diagrams showing impact of implant maskmisalignment on a Mach-Zhedner modulator with mirrored symmetric dopingprofile.

FIG. 3 is a simplified diagram of a Mach-Zhedner modulator with repeatedp-n doping profile associated with two arms according to an embodimentof the present invention.

FIGS. 4A and 4B are simplified diagrams showing impact of implant maskmisalignment on a Mach-Zhedner modulator with repeated symmetric dopingprofile according to an embodiment of the present invention.

FIG. 5 is a simplified diagram showing comparison of phase changes dueto EO effect in two arms of a Mach-Zhedner modulator with mirroredsymmetric doping profile according to a prior art versus a Mach-Zhednermodulator with repeated symmetric doping profile according an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to silicon photonic devices and methods.Various embodiments of the present invention provide a silicon-basedMach-Zhedner modulator with balanced two arms. More specifically, theinvention provides a Mach-Zhedner modulator including repeated p-n orn-p doping pattern to maintain balance for two modulation armsregardless of implant mask misalignment. In certain embodiments, theinvention is applied for high bandwidth optical communication, thoughother applications are possible.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIGS. 1A and 1B are simplified diagrams of typical Mach-Zhednermodulators with two arms formed by mirrored p-n implant masks. TypicalMach-Zhedner (MZ) modulator within silicon photonic platform includestwo identical Si-based waveguides, or Arms A and B. The two Si-basedwaveguides can be made of Si or SiN. Each waveguide includes a PNjunction with a p-type region interfacing a n-type region along thewhole arm length. The p-type region and the n-type region are formed byrespectively doping p- or n-type electric impurity into pre-patternedwaveguide material formed on a silicon-on-insulator (SOI) substrate. Inone example, the p-type region of each PN junction is coupled to anelectrode G_(L) or G_(R) at common ground level (V=0) and the n-typeregion of one PN junction (Arm A) is coupled to an source electrode S+at positive bias level (V>0) while the n-type region of another PNjunction (Arm B) is coupled to another source electrode S− at negativebias level (V<0). The electrode material can be metal or otherconductive material overlying the corresponding p-type or n-type regionsof each PN junction. When two waveguide arms, Arm A and Arm B, arepaired to form the MZ modulator, a central electrode Gc, also at theground level, is added between the two source electrodes S+ and S− butfree from contact either source electrode or part of two PN junctions.Gc is disposed directly on the insulator layer of thesilicon-on-insulator substrate. Adding central electrode Gc makes a sameGSG pattern for each PN junction, forming a GSGSG five-electrodeconfiguration bearing both mirrored and repeated symmetries around eachof the two PN junctions. Further, the two PN junctions of Arm A and ArmB are disposed geometrically in a mirrored symmetric configuration,i.e., either in a p-n n-p or n-p p-n order.

In an application of using push-pull mechanism to drive the MZmodulator, bias voltages applied between the first source electrode S+and ground electrode G_(L) plus a central ground electrode G_(C) togenerate an electric field in GSG pattern for the Arm A. Similarly, biasvoltages applied between the first source electrode S− and groundelectrode G_(R) plus the central ground electrode G_(C) to generate anelectric field in GSG pattern for the Arm B, which is mirrored symmetricto that of Arm A. The waveguide in Arm A or Arm B is configured tocouple with an optical fiber. A beam of light, coming in an opticalfiber, can be split into two by a power splitter and feed intocorresponding two optical fibers coupled to the two waveguides, Arm Aand Arm B. Accordingly, the phases of the beam of light can be properlymodulated in those two arms due to electro-optic (EO) effect under thepush-pull drive mechanism.

In another example, when two modulator arms Arm A and Arm B are pairedto form the MZ modulator, no central electrode Gc is used. Therefore, ontop of mirrored symmetric p-n n-p or n-p p-n junction configuration foreach of the Arm A and Arm B, the associated four electrodes forms a GSSGmirrored symmetric configuration for the MZ modulator. Note, the dopingprocess for forming the PN junction of each arm is typically done byimpurity implantation with a precedent patterning or masking process.Usually p-type doping is done for both PN junctions but is separatelydone for n-type impurity doping. In the prior art, a pair of implantmasks for p-type impurity doing is placed to expose corresponding p-typeregions of the two arms of the MZ modulator with a designated mirroredsymmetric layout. Similarly, another pair of implant masks for n-typedoping is needed to expose corresponding n-type regions in order to formthe required PN junctions. However, a drawback exists with the abovedesign, since any misalignment of either n-type or p-type implant masksover corresponding regions on Arm A and Arm B would result in asymmetricp-n doping profile between the two arms of the MZ modulator, as seen inmore detail below.

FIGS. 2A and 2B are simplified diagrams showing impact of implant maskmisalignment on a Mach-Zhedner modulator with mirrored symmetric dopingprofile. To the left of the figure, a perfect alignment of implant masksis done before performing doping. That results in perfect mirrorsymmetric p-n n-p (or n-p p-n) doping profile so that Arm A and Arm Bare identical. The MZ modulator with such pair of modulation arms isable to generate identical phase changes due to EO effect in the twoarms (A and B), a desired result for utilizing push-pull drivemechanism.

However, to the right of the figure, a situation of misaligned implantmasks is shown. For example, mask for p-type impurity implantation isslightly tilted and/or mask for n-type impurity implantation is slightlyshifted. Any of such misalignment or imperfection of the implant maskingwould result in break-up of mirror symmetry of the doping profilesbetween Arm A and Arm B. Therefore, Arm A and Arm B are no longeridentical, thereby modulation phase changes due to EO effect in Arm Aand Arm B of the MZ modulator are not the same, an undesired result forutilizing push-pull drive mechanism. Since the mirrored symmetricconfiguration is sensitive to the p and n implant masks alignment,improvement on modulation arms configuration is needed.

FIG. 3 is a simplified diagram of a Mach-Zhedner modulator with repeatedp-n doping profile associated with two arms according to an embodimentof the present invention. This diagram is merely an example, whichshould not unduly limit the scope of the claims. One of ordinary skillin the art would recognize many variations, alternatives, andmodifications. As shown, a repeated, not mirrored, symmetric p-type andn-type implant masks are provided. When the corresponding implant masksin repeated symmetric pattern are utilized for doping p-type or n-typeimpurities in the two silicon-based waveguides, two PN junctions withrepeated symmetry are formed. The implantation process is performed fordoping p-type impurity at one time into corresponding p-type regions ofboth waveguides based on the p-type implant masks with repeatedsymmetry. Then, the implantation process is performed for doping n-typeimpurity at another time into corresponding n-type regions of bothwaveguides based on the n-type implant masks with repeated symmetry.

Additionally, to form two balanced modulator arms with repeatedsymmetry, the corresponding electrode setting is also required to beformed in repeated symmetric pattern. Particularly for the embodimentshown in FIG. 3, a five-electrode GSGSG configuration in a directionacross the two modulator arms is utilized. As shown, at least from onedirection across the two modulator arms, a first part of a common groundelectrode, G_(L), is disposed to couple with the p-type (or n-type)region of a first PN junction of a first modulator arm (e.g., Arm A′)followed by a first source electrode (S+) disposed to couple with then-type (or p-type) region the same PN junction of the same modulatorarm. Further along this direction, a second part of the common groundelectrode (G_(C)) and a second source electrode (S−) are disposed inrepeated symmetry to couple respectively with the second PN junction ofthe second modulator arm (e.g., Arm B′), followed by a third part of thecommon ground electrode (G_(R)). The second part of the commonelectrode, namely a central ground electrode (G_(C)), of course has aspacing away from the first PN junction and similarly in a repeatedsymmetry the third part of the common ground electrode has a spacingaway from the second PN junction. The three parts of common groundelectrode are at a same grounded state (V=0) while the first sourceelectrode and the second source electrode are biased to a voltage havinga same value but opposite in positive/negative polarity. In this way,each modulation arm, Arm A′ or Arm B′, is subjected to same alternatingelectric field induced by the bias voltages applied via GSG electrodesin repeated symmetric configuration. Detail illustration is shown belowabout how this repeated symmetry in the formation of both the two PNjunctions and overlying electrodes improves the balance of two arms ofthe MZ modulator.

FIGS. 4A and 4B are simplified diagrams showing impact of implant maskmisalignment on a Mach-Zhedner modulator with repeated symmetric dopingprofile according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown, to the left ofthe figure, p-type and n-type implant masks are perfectly aligned in ArmA′ and Arm B′. Of course, there will be no issue at all since the p-ndoping profiles of the Arm A′ and Arm B′ are identical in this situationand the resulted phase changes the two arms of MZ modulator are alsoidentical. Alternative, to the right of the figure, a scenario ofmisaligned implant masks is shown. However, since either p-type implantmask or n-type implant mask is repeatedly applied to Arm A′ and Arm B′,any misalignment or shifting relative to a central line of each arm isstill repeated for both Arm A′ and Arm B′. Therefore, corresponding p-ndoping profiles associated with both Arm A′ and Arm B′, although notperfect by itself, remain identical among each other. Accordingly, themodulation phase changes due to EO effect in both Arm A′ and Arm B′ of aMZ modulator are still physically identical. Thus the push-pull drivemechanism can be implemented without being affected by the manufactureimperfection.

Similarly, the electrodes coupled with the p or n regions of each PNjunction can be formed with repeated symmetry. Metal or other conductivematerials can be deposited or plated onto pre-patterned regions foreither grounded electrode or source electrode with repeated symmetry forthe two arms of the MZ modulator. Though not shown in FIG. 4, theresults would be similar to the formation of PN junctions as manufactureprocess is much easier to achieve repeated symmetric patterns thancreating mirrored symmetric patterns.

FIG. 5 is a simplified diagram showing comparison of phase changes dueto EO effect in two arms of a Mach-Zhedner modulator with mirroredsymmetric doping profile according to a prior art versus a Mach-Zhednermodulator with repeated symmetric doping profile according an embodimentof the present invention. This diagram is merely an example, whichshould not unduly limit the scope of the claims. One of ordinary skillin the art would recognize many variations, alternatives, andmodifications. As shown, the phase changes of Mach-Zhedner modulator aresubstantially linear over the applied bias voltages. To the left part ofthe figure, phase changes of two arms of a MZ modulator with mirroredsymmetric doping profile are plotted. Due to broken symmetry caused bymanufacture misalignment or imperfect doping profile variations, the PNjunction in one arm (Arm A) is no longer identical to that of anotherarm (Arm B). This results in a reduced slope of the linear plot for thephase change of Arm B.

On the other hand, to the right part of the figure, the phase changes ofa MZ modulator with two arms in repeated symmetric doping profileaccording to an embodiment of the present invention are plotted againstthe applied bias voltages, keeping substantially the same linearrelation for the two arms, Arm A′ and Arm B′. The repeated symmetry ofthe PN junctions for the two arms is not affected at all by anymisalignment of corresponding p-type or n-type implant mask and resultedimperfect doping profiles within each arm, either Arm A′ or Arm B′. Inother words, the PN junction in Arm A′ remains identical to that in ArmB′. When a beam of light is split into the two arms, phase changes inArm A′ and Arm B′ are the same so that the push-pull phase modulationmechanism can be properly executed for the MZ modulator.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A Mach Zehnder (MZ) modulator comprising: a firstwaveguide in a linear shape having a first p-type side joined with afirst n-type side along a first direction on a silicon-on-insulatorsubstrate; a second waveguide in a linear shape having a second p-typeside joined with a second n-type side substantially parallel to thefirst waveguide along the first direction on the silicon-on-insulatorsubstrate, the second waveguide being displaced from the first waveguideby a distance along a second direction perpendicular to the firstdirection, wherein the second p-type side joined with a second n-typeside is substantially a replica of the first p-type side joined with thefirst n-type side having a same p-n p-n or n-p n-p order along thesecond direction; a first electrode and a second electrode disposedseparately on the first waveguide to respectively couple to the firstp-type side or the first n-type side; a third electrode and a fourthelectrode disposed separately on the second waveguide to respectivelycouple to the second p-type side or the second n-type side, the thirdelectrode and the first electrode being commonly coupled to ground, thesecond electrode being located between the first electrode and the thirdelectrode; and a fifth electrode disposed outside the second waveguidemore distal to the first waveguide, wherein the third electrode iscommonly coupled to ground with the first electrode and the thirdelectrode, the second electrode and the fourth electrode are sourceelectrodes respectively located in a first position between the firstelectrode and the third electrode and a second position between thethird electrode and the fifth electrode, thereby having the firstwaveguide and the second waveguide subjected to a substantially sameelectric field varied in a ground-source-ground pattern.
 2. The MZmodulator of claim 1 wherein the first p-type side of the firstwaveguide and the second p-type side of the second waveguide are inparallel to each other, the first n-type side of the first waveguide andthe second n-type side of the second waveguide are in parallel to eachother.
 3. The MZ modulator of claim 1 wherein the first waveguide andthe second waveguide are made of Si or SiN patterned within thesilicon-on-insulator substrate.
 4. The MZ modulator of claim 1 whereinthe second electrode and the fourth electrode are source electrodesalternatively set at a positive and a negative bias voltage or viceversa relative to ground.
 5. The MZ modulator of claim 1 furthercomprising a power splitter to split a beam of light via a first opticalfiber and a second optical fiber respectively coupled into the firstwaveguide and the second waveguide.
 6. The MZ modulator of claim 5wherein the first waveguide is configured to provide a first phasemodulation to a first split portion of the beam of light driven byelectric field through a first voltage biased at the second electrodesandwiched by the first electrode and the third electrode commonly inground, the second waveguide is configured to provide a second phasemodulation to to a second split portion of the beam of light driven byelectric field imposed a second voltage biased at the fourth electrodesandwiched by the third electrode and the fifth electrode commonly inground.
 7. The MZ modulator of claim 6 wherein the first voltage and thesecond voltage are equal in value but alternative in polarity.
 8. The MZmodulator of claim 6 wherein the beam of light is an output of a lasersource.
 9. A method for manufacturing a linear Mach Zehnder modulatorwith balanced arms, the method comprising: providing asilicon-on-insulator substrate; forming a first linear waveguide and asecond linear waveguide in parallel in the silicon-on-insulatorsubstrate, the second linear waveguide being separated from the firstlinear waveguide by a distance in perpendicular direction; forming afirst PN junction and a second PN junction respectively in the firstlinear waveguide and the second linear waveguide, the first PN junctionand the second PN junction having a substantially identical repeated p-np-n or n-p n-p doping profile along the perpendicular direction; forminga first electrode and a second electrode to couple with the first PNjunction and forming a third electrode and a fourth electrode separatelyto couple with the second PN junction; and forming a fifth electrodebeyond a side of the second linear waveguide distal to the first linearwaveguide, the fifth electrode being commonly grounded with both thefirst electrode and the third electrode, the second electrode and thefourth electrode are source electrodes respectively located in a firstposition between the first electrode and the third electrode and asecond position between the third electrode and the fifth electrode forimposing a substantially same electric field varied in aground-source-ground pattern across each of the first PN junction and asecond PN junction.
 10. The method of claim 9 wherein forming the firstlinear waveguide and the second linear waveguide comprise patterning,depositing, or doping Si or SiN within the silicon-on-insulatorsubstrate.
 11. The method of claim 9 wherein forming the first PNjunction and the second PN junction comprising implanting p-type/n-typeimpurity into a first side region in each of the first linear waveguideand the second linear waveguide at a first time followed by implantingn-type/p-type impurity to a second side region in each of the firstlinear waveguide and the second linear waveguide at a second time,wherein the second side region of the first linear waveguide is at leastpartially joined with the first side region of the first linearwaveguide along a length direction, wherein in a repeated symmetryrelative to the first linear waveguide the second side region of thesecond linear waveguide is at least partially joined with the first sideregion of the second linear waveguide along the length direction. 12.The method of claim 11 wherein forming a first electrode and the secondelectrode comprises coupling the first electrode to the first sideregion of the first linear waveguide and coupling the second electrodeto the second side region of the first linear waveguide.
 13. The methodof claim 11 wherein forming a third electrode and a fourth electrodecomprises coupling the third electrode to the first side region of thesecond linear waveguide and coupling the fourth electrode to the secondside region of the second linear waveguide.
 14. A method for balancingtwo arms of a Mach-Zehnder modulator, the method comprising: providing asilicon-on-insulator substrate; forming a first linear waveguide and asecond linear waveguide in parallel in the silicon-on-insulatorsubstrate, the second linear waveguide being shifted a distance from thefirst linear waveguide in a direction perpendicular to the first linearwaveguide; masking a first side region of the first linear waveguide anda second side region of the second linear waveguide with repeatedsymmetry; implanting p-type impurity to the first side region and thesecond side region; masking a third side region of the first linearwaveguide and a fourth side region of the second linear waveguide withrepeated symmetry; implanting n-type impurity to the third side regionand the fourth side region, wherein the third side region at leastpartially joins with the first side region to form a first PN junctionin the first linear waveguide and the fourth side region at leastpartially joins with the second side region to form a second PN junctionin the second linear waveguide, wherein the first PN junction and thesecond PN junction comprise either p-n p-n or n-p n-p doping profile inrepeated symmetry; forming a first electrode and a second electroderespectively coupled to first side region and the third side regionassociated with the first PN junction; forming a third electrode and afourth electrode respectively coupled to the second side region and thefourth side region associated with the second PN junction, the thirdelectrode and the fourth electrode being respectively shifted thedistance from the first electrode and from the second electrode; andforming a fifth electrode at a position shifted the distance from thethird electrode beyond the second PN junction, the fifth electrode beingcommonly grounded with both the first electrode and the third electrode,the second electrode and the fourth electrode being source electrodesrespectively located in a first position between the first electrode andthe third electrode and a second position between the third electrodeand the fifth electrode for imposing a substantially same electric fieldvaried in a ground-source-ground pattern across each of the first PNjunction and a second PN junction.
 15. The method of claim 14 whereinthe first linear waveguide and the second linear waveguide comprise Sior SiN patterned within the silicon-on-insulator substrate.
 16. Themethod of claim 14 wherein forming a fifth electrode comprises forming acombination of the third electrode, the fourth electrode, and the fifthelectrode in a repeated symmetry of a combination of the firstelectrode, the second electrode and the third electrode.
 17. The methodof claim 14 wherein the second electrode and the fourth electrode areconfigured to be biased with a same voltage level but opposite inpolarity relative to ground.